Esther Inglis-Arkell

Folie à deux is a term that originated in psychiatry, but sounds charming and whimsical enough that it's been used as title for everything from novels to wineries. Psychiatry has replaced the term with "shared psychotic disorder," which is a much less pretty, but much more accurate, description of the condition.

The French first coined the phrase to describe the case of Margaret and Michael, a married couple. The doctors didn't know which of the two started the cycle of psychosis, but they got into a feedback loop that reinforced each other's delusions. They both believed that their home was being targeted by random people. These people never stole or wrecked the place, according to the delusional couple. Instead, these interlopers would spread dust around the house, take up pieces of lint and scatter them, and either grind or walk in both the couple's shoes until the soles were worn down.

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Husbands and wives are one of the relationships commonly seen in shared psychotic disorder. One case report has a husband and wife agreeing that she had cheated on him hundreds of times, sometimes within a timeframe of two weeks. Each time she committed these acts she had amnesia, and didn't remember any of them. When a psychologist expressed disbelief the husband became extremely angry, and accused the psychologist of having an affair with his wife as well.

It's also common in sibling relationships. A recent, and sensational, case of the disorder occurred in 2008, when two sisters, Ursula and Sabina Eriksson, were asked to leave a bus after they refused to allow their luggage to be searched. Police briefly talked to the sisters, but allowed them to leave — and that's when the women tried to dart into traffic. When patrol cars arrived on the scene, Ursula suddenly ran into the street. Before the police could respond, Sabina did the same thing. They fought the police and medical teams trying to help them, screaming, "I know you're not real," and "They're trying to steal our organs." Sabina was briefly jailed for striking a police officer. When she got out of jail, she wandered around looking for her sister, and eventually stabbed a man to death when he tried to help her. In the frenzy of coverage that followed the murder, it was discovered that the twins had a brother. When he was contacted he told the press that they were "fleeing from maniacs."

Three American sisters, none of whom had any history of abuse or mental difficulty, ended up starring in another spectacular instance of shared psychotic disorder - folie à trois, in this case. At first, the three sisters came together because the eldest needed help taking care of her children. The eldest lived with her husband, and the other two sisters moved into a house nearby, helping out whenever she needed them. Over time, the sisters became closer, and became more religious. Eventually, the youngest sister believed that there was some kind of problem with the Bible, because of the different spellings from version to version, and that she was chosen to make it right. After three days of continuous prayer, without sleep, the sisters decided that she needed to start her work in a specific house, which belonged to someone else. They traveled to the house and demanded to be let in to "their" room. When they were refused, the first tried to break into the house, and then they attacked the police officers who came to stop them. They were put in the same holding cell in the jail, where they continually prayed and sang, naked, and occasionally attacked the guards.

Although pre-existing disorders like depression, delusions, or paranoia can spark a folie à deux, all that's really necessary is social isolation and a difficult-to-break connection between the two people. This is why almost all such cases happen in families. Couples account for seventy percent of the cases. Siblings and parent-child relationships make up most of the rest. Although the overall disorder is split evenly between male and female patients, sisters tend toward shared psychotic disorders more than other siblings.

How to combat it? In the case of the sisters, all they needed was some time away from each other. When they were separated, they gave up their disorders without any medication. They were banned from seeing each other without supervision, banned from living too near each other, and banned from living near the person whose house they attempted to break into. This was a truly shared delusion. Other times, the delusion is imposed by a more dominant partner, who may not shake it as well as the subordinate one does. There are also cases when the subordinate partner picks up new delusions, or clings to them long after the dominant one has stopped. Generally, doctors deal with extreme cases by prescribing anti-psychotic medications.

One of the major fascinations of folie à deux is the fact that we all do it, to a certain extent. There isn't anyone who isn't shaped by their parents, their siblings, or their loved ones. Ideally, this shaping is a positive experience. It's not hard to see how terribly this can go wrong for anyone. There are few old married who don't share eccentricities. There are few families, or even close friendships, that don't require both people to work with the various mental glitches of the other. We all go a little crazy for the other people in our lives.